The present invention relates to a light emitting diode device having a light emitting diode (LED) as a light source.
In recent years, there has been provided an LED device which may emit white light, and furthermore, an LED device which may emit light of various colors has been proposed.
FIG. 18 is a perspective view of a conventional LED device for emitting white light, FIG. 19 is a sectional view of the LED device.
The white light emitting LED device 120 comprises a substrate 101 having a pair of terminal electrodes 102 and 103 provided on the upper surface and the underside thereof, and an LED 105 for emitting blue or ultraviolet light. The cathode of the LED 105 is connected to the electrode 102 through an adhesive 104, and the anode is connected to the electrode 103 by a bonding wire 106. The LED 105 and the upper surface of the substrate 101 are covered by a transparent resin 107. In the resin 107, yellow phosphor particles 108 are mixed.
When a driving voltage is applied to the terminal electrodes 102 and 103, the LED 105 is excited to emit blue or ultraviolet light S as shown in FIG. 20.
When the blue or ultraviolet light S strikes the phosphor particle 108, the phosphor particle emits yellow light or green light excited from red-green-blue S1. The mixture of the bluish light and the yellowish light takes on white light based on the wavelength conversion.
FIG. 21 is a sectional view showing another conventional LED device 130. The same parts as the conventional LED device of FIGS. 18 and 19 are identified by the same reference numerals as those of FIGS. 18 and 19. In the resin 107, colored particles 109 are mixed as color filters.
The white light by the mixture of the bluish light and the yellowish light described above is changed by the color of the colored particle 109 by the subtractive color mixing consequently, by selecting the color of the colored particles 109, desired color light is produced. Thus, the LED device 130 is provided to produce various color light.
FIG. 22 is a perspective view showing a back light unit for illuminating an LCD (liquid crystal display), FIG. 23 is a sectional view of the back light unit.
The back light unit 140 comprises a pair of white light emitting LED devices 120 shown in FIG. 18, a lighting panel 142, a diffusion panel 143, a Py prism sheet 144, a Px prism sheet 145, a reflection plate 146, and a color LCD 147.
The lighting panel 142 is made of a transparent plastic and has an upper surface 142a, lower surface 142b and front side 142c. The white light emitting LED devices 120 are mounted on an LED substrate 120b and disposed opposite the front side 142c as edge light. The diffusion panel 143 is disposed above the upper surface 142a of the lighting panel 142 and the reflection plate 146 is disposed below the lower surface 142b. 
The white light emitted from the LED devices 120 enters the lighting panel 142 from the front side 142c. The entered light is repeatedly reflected by the upper and lower surfaces 142a and 142b. The light is diffusely reflected by the prism surface of the lower surface 142b and discharged from the upper surface 142a. Instead of prism, a crease or uneven surface may be used.
The discharged direction of the light is arranged in a small range by the diffusion panel 143, and further arranged by the prism sheets 144 and 145 in the Y and X-directions, and finally arranged in the Z-direction. The light arranged in the Z-direction illuminates the LCD 147.
In such an illuminating device, LCDs vary in the characteristic of the color filter provided therein. Namely, the color filter characteristic varies with the manufacturer.
On the other hand, the chromaticity of the picture displayed on the LCD is determined by the characteristic of the color filter and the chromaticity of the white LED device 120 illuminating the LCD. The relationship between the characteristics will be described with reference to a drawing hereinafter.
FIG. 24 is a graph of CIE chromaticity. Here, x-coordinate designates proportion of R (Red), y-coordinate designates proportion of G (Green). If proportion of B (Blue) is designated by z, there is the following relationship there-between.x+y+z=1
The point c0 in the graph is chromaticity point where the ratio of R, G and B is 1:1:1.
Coordinates of the point c0 are about x=0.33, y=0.33, z=0.33. The point b0 is a coordinate point of an aim chromaticity of the white LED device, and the reference letter B designates an allowable range of the point b0. The coordinates of the point b0 are x=0.313 and y=0.308. The point d0 is a chromaticity point of the wavelength transmittance of the color filter of the LCD. The coordinates are x=0.352 and y=0.357. The point d0 has a complementary color relation to the point b0. The reference letter D designates a dispersion range of the point d0.
If the chromaticity of the white LED device 120 is the chromaticity at the point b0 or a value in the allowable range B, the light from the white LED device 120 transmits the color filter having a chromaticity of the point d0 or a value in the range D, so that the light is corrected to light having a chromaticity based on the white light designated at the point c0.
However, the chromaticity of the light emitted from the white LED device 120 varies according to dispersion of the wavelength and intensity of the light emitted from the LED 105 and dispersion of the distribution of particles in the resin 107 of the product.
The average values of coordinates of actually manufactured white LED devices are x=0.295 and y=0.290, and the dispersion is σx=0.015 and σy=0.01.
The point f0 in FIG. 24 designates an average chromaticity of the above described products. F designates a range of the dispersion. Therefore, actual products each having chromaticity in the desired range B is a very small percentage of all products. Thus, the yield of the white LED device is very low.